Two sequential percentage discounts in the same formula

I need to take a discount off of a previously applied discount. For example:

I buy something for $50, then take 10% off,the formula I have for that is: (50-(50*.1))

That's pretty simple. However, I need to take another discount off of that result, and that's the part where I get confused.

When doing it on the calculator, the result is $40.50. I can't simply add the two discounts because that would be 20% off which equals $40.00, so I'm trying to figure out how to write this formula so that the 2nd discount takes into account the result of the first discount.

Any ideas?

Awesome! Thanks for this,works great in my formula now!